Characterization of thermal diffusion in polymer solutions by thermal field‐flow fractionation: Dependence on polymer and solvent parameters

Abstract

SynopsisThe thermal diffusion coefficient DT has been obtained for 17 polymer-solvent combinations, each of them spanning a range of polymer molecular weights, using thermal field-flow fractionation. The polymers examined include polystyrene, poly(alpha-methyl)styrene, polymethylmethacrylate, and polyisoprene. The solvents include benzene, toluene, ethylbenzene, tetrahydrofuran, methylethylketone, ethylacetate, and cyclohexane. Although D, was confirmed as essentially independent of polymer molecular weight, it was found to vary substantially with the chemical composition of polymer and solvent. The results were used to evaluate several thermal diffusion theories; the agreement with theory was generally found to be unsatisfactory. Attempts were then made to correlate the measured thermal diffusion coefficients with various physicochemical parameters of the polymers and solvent. A good correlation was found in which D, increases with the thermal conductivity difference of the polymer and solvent and varies inversely with the activation energy of viscous flow of the solvent.

Determination of the compositional heterogeneity of polydisperse polymer samples by the coupling of size-exclusion chromatography and thermal field-flow fractionation

“…8. In agreement with the measuzements of Schimpf and Giddings [33], a mass independence of the thermal diffusion coefficient was found. The mean DT value of 0.92' 10 -7 cm 2 s -t K -l (the dashed line in Fig.…”

“…For the PMMA homopolymer an approximately constant D T value of 7 2 1 1 1.39.10-cm s-K-was found. This value is in good agreement with the thermal diffusion coefficient of 1.33.10 -7 cm 2 s -l K -t, that has been reported for the PMMA-THF system [33]. It should be noted that for this analysis a slight increase in D T was found going from fraction 2 to fraction 8.…”

“…The mean DT value of 0.92' 10 -7 cm 2 s -t K -l (the dashed line in Fig. 8) is in reasonable a~reement with the value of 1.00.10 -'t cm 2 s -1 K-" thet has been reported in literature [33]. The constant D T value indicates that no significant systematic errors occurred during the SEC-ThFFF cross-fractionation.…”

“…Retention in :~'hFFF is determined by the applied temperature drop and the ratio of the molecular and thermal diffusion coefficient of the polymer. Although no real understanding of the thermal diffusion effect in li~uids exists, ThFFF studies have indicated that the thermal diffusion coefficient is indepen-dent of the molecular mass of the polymers, whereas it is strongly affected by the chemical composition of both the polymer and the solvent [33]. This latter statement indicates that ThFFF also has potentials to fractionate polymer and copolymer samples according to chemical composition.…”

A new analytical formulation of retention effects on particle diffusion processes

“…The extended theory including thermodynamic coupling has still to be developed. Classical approaches can be found in the current literature (Kirkland et al 1990, Eon et al 1997, Fitch et al 1996, Schimpf and Giddings 1989, but there is no satisfactory theory including retention effects. The determination of the functional dependence of β on mass concentration, on temperature and on other intervening molecular forces is, for instance, a challenge for further investigation.…”